1. Technical Field
This invention relates to adaptive multibeam antenna communication systems, and more particularly to an adaptive multibeam antenna system for constraining pulse jammer duty factors.
2. Discussion
Various types of adaptive antenna control systems have been developed to counteract jamming signals. In general, adaptive antenna control systems respond to a jamming signal by forming a nulling signal which, when combined with the signals received by the antenna, greatly reduces the gain in the direction (channel) in which the jamming signal appears.
A number of mathematical techniques, known as nulling algorithms, have been used to analyze jamming signals and produce effective nulls. These algorithms each have advantages and disadvantages depending on system factors such as the antenna, the noise environment, and the communication link. For example, a multibeam antenna, as opposed to a phased array, presents a non-uniform distribution of power to the feed elements. The use of scaling based on measured power levels, together with eigenvalue shifting, is advantageous with multibeam antennas because it exploits the antenna's non-uniform distribution of power to produce faster nulls with simple algorithms. During nulling, the communication link requires protection against jammer bursts. Coding techniques such as forward error correction coding with interleaving can provide this protection only if the duty factor of the high power bursts is less than a critical value. This critical value is a function of coding complexity and modulation format. Since the nulling antenna's output jammer power is the product of the received jammer power and antenna gain in the jammer's direction, the relationship between gain and duty factor constrains the effect of jammer bursts on the bit error rate performance of the link. To obtain a small duty factor of the high gain state presented to the jammer by the adaptive antenna requires a fast-attack/slow-release nulling transient response.
Direct approaches, such as direct matrix inversion and conjugate-gradient methods, lack this "null memory" characteristic and recover too quickly. Fixed step-size gradient algorithms are too sensitive to eigenvalue spread and converge slowly if the eigenvalue ratio is large. Iterative algorithms based on relaxation offer compromises that appear well suited to realistic jammer threats. They feature scaling and eigenvalue shifting, which reduces the condition number of the iteration matrix so that the eigenmodes converge at consistent speeds when forming nulls. Thresholding the scaling and eigenvalue shifting operation is a modification that preserves the adaptive null memory to accommodate interleaved burst-error coding requirements.
Blinking jammers may try pseudo random on-off modulation and power level control strategies. Real-time scaling, at the iteration rate, combats non-stationary power levels and retains consistent "fast-attack" characteristics. Adaptive step-size control further optimizes the convergence speed of the algorithm. Forward error-correction codes need interleaving to protect the communication link against burst errors. Faster nulling reduces burst lengths and therefore requires a smaller interleaver to protect the communication link until the adaptive antenna forms nulls. The combination of nulling and coded spread-spectrum processing provides substantial jammer protection. Once the null is formed, further jamming is useless and will soon be discontinued.
In the classic burst channel, the period between jammer bursts when the channel is error-free, is called the guardband. Rate - one half Reed -Solomon codes and interleavers can be designed to correct all burst errors with a guardband/burst-duration ratio as small as three. However, rate - 1/2 Viterbi-decoded convolutional code using pseudo-random interleaving lose only one dB of margin for duty cycles of 10% or less with erasure decoding and 1% for errors-only decoding. My analyses of other, practical, burst-coded channels show similar results, except that a trade-off is allowed between power level and duty-factor. "Slow release" of null patterns insures a guardband of low gain between jammer bursts sufficient to keep the duty-cycles in this range. Fast-attack/slow-release transient characteristics depend on retaining a spread between eigenvalues of the iteration matrix in the on-off and off-states of the blinking jammer. This spread is naturally present in fixed step-size algorithms, but the attack-time can be better controlled in more advanced algorithms by scaling, eigenvalue shifting, and adaptive step-size. Since nulling also reduces the service area of coverage to users, it is more efficient to allow some margin for low powered interference and apply scaling only if the noise power level exceeds a given threshold.
Thus, it would be desirable to provide an adaptive nulling antenna control system which can provide effective nulling against blinking jammers. In this regard, it would be desirable to provide such a nulling antenna control system which can produce nulls very quickly and maintain the null relatively long to provide a fast attack and slow release performance. Further, it would be desirable to provide such an nulling antenna control system which utilizes an iterative algorithm incorporating real-time scaling and eigenvalue shifting with thresholding and adaptive step-size control to optimize convergence speed. Further, it would be desirable to provide such a system which is relatively straightforward to implement and which can take advantage of parallel processors to further increase convergence speed.